Recently, nonaqueous electrolyte batteries such as a lithium ion secondary battery have been developed as a battery having a high energy density. The nonaqueous electrolyte battery is expected to be a power source of vehicles such as a hybrid automobile or an electric automobile, or a power source for a large-sized storage battery. In particular, with respect to the application for vehicles, it is required for the nonaqueous electrolyte battery to have other properties such as a rapid charge-and-discharge performance and a long term reliability. A nonaqueous electrolyte battery capable of rapid charge-and-discharge has an advantage of remarkably short discharge time. The nonaqueous electrolyte battery capable of rapid charge-and-discharge can improve a power performance of a hybrid automobile and can efficiently recover a regenerative energy of the power.
The rapid charge-and-discharge can be performed in a manner in which electrons and lithium ions rapidly move between a positive electrode and a negative electrode. In a battery using a carbon negative electrode, a dendrite of metal lithium may sometimes be deposited on an electrode by repeating the rapid charge-and-discharge. The dendrite causes an internal short-circuit, thus resulting in a risk of generation of heat or ignition.
Therefore, a battery in which a metal composite oxide is used as the negative electrode active material instead of a carbonaceous material has been developed. In particular, a battery using a titanium oxide as the negative electrode active material can stably perform the rapid charge-and-discharge, and has a characteristic of a longer life time compared to that of the carbon negative electrode.
The titanium oxide, however, has a potential higher (nobler) than that of the carbonaceous substance to metal lithium. In addition, the titanium oxide has a low capacity per weight. A battery using titanium oxide, accordingly, has a defect of a low energy density.
For example, the electrode potential of the titanium oxide is about 1.5 V vs. metal lithium, which is higher (nobler) than that of the carbon negative electrode. The potential of the titanium oxide is caused by an oxidation-reduction reaction between Ti3+ and Ti4+ when lithium is electrochemically inserted and extracted, and thus it is electrochemically limited. There is also the fact that the rapid charge-and-discharge of the lithium ions can be stably performed at a high electrode potential of about 1.5 V. In order to improve the energy density, accordingly, it is substantially difficult to decrease the electrode potential.
On the other hand, with respect to a capacity per unit weight, a lithium-titanium composite oxide such as Li4Ti5O12 has a theoretical capacity of about 175 mAh/g; whereas a generally used graphite electrode material has a theoretical capacity of 372 mAh/g. The capacity density of the titanium oxide, accordingly, is remarkably lower than that of the carbon negative electrode material. This can be caused due to the small number of sites for inserting lithium in the crystal structure of the titanium oxide, or a decrease in the substantial capacity because lithium is easily stabilized in the structure.
In view of the above, a novel electrode material containing Ti and Nb is studied, in particular, a monoclinic Nb—Ti composite oxide, represented by TiNb2O7, is receiving attention because charge compensation, in which tetravalent Ti is turned into trivalent Ti, and pentavalent Nb is turned into trivalent Nb, is caused upon the Li insertion, and thus a high theoretical capacity such as 387 mAh/g can be obtained.